Question

waves on the surface of a liquid are observed to have a wavelength of 12.9 mm and a speed of 30.9 cm/s. the frequency of the wave motion is:

a. 1.8 mHz
b. 24 Hz
c. 42 mHz
d. 2.4 Hz
e. 2.4 mHz

how do i know which label to use (mHz or Hz)? and which one is correct?